1. Technical Field of the Invention
This invention relates to an adjustment method for an active-matrix type liquid crystal display device with multiple pixels arranged in a matrix of columns and rows.
2. Related Art
In recent years, because of their thinness, light weight and low power consumption, matrix type liquid crystal display devices have been used in various fields such as display devices for personal computers and word processors, TV display devices, and also projection type display devices.
Amongst these, active-matrix liquid crystal display devices include switching elements electrically connected to pixel electrodes to drive liquid crystal layers and achieve fine display images without cross-talk between neighboring pixels. Therefore, research and development of these devices has been vigorously carried out.
However, in such an active-matrix type liquid crystal display device, when a DC voltage is applied to the liquid crystal layer for a long time, this leads to deterioration of the liquid crystal material and makes it difficult to keep display images in good quality for a long period of times. To avoid these technical difficulties "frame reversal drive" is used. Generally, at every single vertical scanning period it reverses the polarities of potential differences (voltages) to be applied to the liquid crystal layers through pixel and counter electrodes therebetween.
Also, to prevent flickering of the display screen, the technique of reversing the polarities of potential differences applied to the liquid crystal layers at every single vertical scanning period and, at the same time, reversing the same every pixel or every scanning line has become known through, for instance, Japanese Laid-Open Patent Application Nos. 61-275822 and 62-218943.
In short, as shown in FIG. 15(a) and (15b), a "horizontal (H) line reversal driver" is used in which, in addition to the reversal of the polarities of the potential differences applied to the liquid crystal layers (through pixel and counter electrodes connected therebetween) every single vertical scanning period, the polarities of the potential differences applied to the liquid crystal layers are also reversed at every single horizontal pixel line or every multiple of neighboring horizontal pixel lines. Also, as shown in FIG. 16(a) and 16(b), a "vertical (V) line reversal drive" is used in which, in addition to the reversal of the polarities of the potential differences applied to the crystal layers at every single vertical scanning period, the polarities of the potential differences applied to the liquid crystal layers are also reversed at every single vertical pixel line or every multiple of neighboring vertical pixel lines. Further, as shown in FIGS. 17(a) and 17(b), a "dot (HV) reversal drive" is used in which, in addition to the reversal of the polarities of the potential differences applied to the crystal layers every single vertical scanning period, the polarities of the potential differences applied to the liquid crystal layers are also reversed at every single pixel or every multiple of neighboring pixels.
As shown in FIG. 18, a pixel electrode E of a liquid crystal layer is connected to a thin-film transistor (hereafter abbreviated as "TFT") in the vicinity of the crossing of signal line Xi and scanning line Yj for every pixel of an active-matrix type liquid crystal display device. Counter electrode C of the liquid crystal layer is disposed opposite to this pixel electrode E. Moreover, in order to suppress the fluctuation of the pixel electrode voltage, supplementary electric capacitor Cs is connected in parallel with equivalent electric capacitor Clc to the liquid crystal layer (a reference voltage is supplied to line CS connected to the capacitor Cs).
With this structure, a parasitic electric capacitance Cgs unavoidably exists between the gate and source electrodes of the TFT and also between signal line X and pixel electrode E. For this reason, when the TFT operates as n-type, pixel electrode voltage Ve is applied to parasitic capacitance Cgs simultaneously with the turning off of the TFT, and pixel electrode voltage Ve shifts level to the negative side. In the event that the TFT operates as p-type however, the voltage polarity thereof is reversed in the present case.
A positive polarity is when the pixel electrode voltage is higher than the counter electrode voltage while a negative polarity is when the pixel electrode voltage is lower than the counter electrode voltage.
To prevent from flickering and applying DC voltage to the liquid crystal layers and to keep display images in good quality, relative potential differences between counter voltage Vcom and image signal voltage Vsig should be determined essentially in consideration of level shift at the pixel electrode resulting from the stray capacitance Cgs.
Such level shift depends on the values of equivalent liquid crystal capacitor Clc, supplementary capacitor Cs and parasitic capacitance Cgs which vary from product to product. As a result, relative potential differences between counter electrode voltage Vcom and video signal voltage Vsig cannot be precisely determined in advance through engineering design.
Therefore, normally, adjustment is performed visually to reduce flicker by an operator in a state in which an image of the same brightness is displayed over the whole screen. However, in the above active-matrix type liquid crystal display devices which are V-line reversal driven, H-line reversal driven or HV-reversal driven, compared with normal frame-reversal drive, the drive is such that the polarity reversal cycle is short and flicker is hardly noticeable. Therefore, strict visual adjustment is difficult.
For this reason, although there appears to be no flicker problem with the displayed image, the liquid crystal layer will receive DC voltage for a long time due to improper adjustment of the positive and negative potential differences. Therefore, the variation in life span occurs from product to product.